This invention relates to communication systems, and more particularly, this invention relates to high frequency signal sources using dielectric resonator oscillators (DRO""s) used for producing a local oscillator signal.
In communication circuits, the Local Oscillator (LO) signal must exhibit low phase noise, corresponding to the random phase instability of a signal, to meet the signal requirements of the digital modulation scheme used in a communications system. The signal also must maintain acceptable bit error rate (BER) requirements. Any circuit that generates the local oscillator signal should have a wide tuning range for multiple channel selection, and a minimal frequency drift over temperature to maintain the transceiver locked into a selected channel. It should provide enough power to drive directly a mixer for up or down frequency conversion as required in modern, high frequency communications systems.
Free running Dielectric Resonator Oscillators (DRO) are attractive, high frequency microwave sources because of their high Q, low phase noise, good output power and a relatively high stability of operation versus temperature. These type of oscillators represent a good compromise of cost, size, and performance compared to alternative prior art signal sources, such as cavity oscillators or multiplied crystal oscillators. Dielectric resonator oscillators, however, do not have any controllable tuning range, and they exhibit frequency drift versus temperature and aging. Also, these devices have always required manual tuning to achieve the desired frequency accuracy.
Voltage controlled Dielectric Resonator Oscillators (DRO) are similar devices that are also attractive microwave sources because of their high Q, low phase noise, good output power and high stability versus temperature. These devices also represent a fair compromise of cost, size, and performance compared to cavity oscillators and multiplied crystal oscillators. Drawbacks of voltage controlled dielectric resonator oscillators are limited electronic tuning range and have a reduced Q because of varactor pulling. They also exhibit frequency drift versus temperature and aging. As a result, these dielectric resonator oscillators have always required manual tuning to achieve the desired frequency accuracy.
It is therefore an object of the present invention to provide a high frequency signal source and method using a Dielectric Resonator Oscillator (DRO) circuit that overcomes the disadvantages of the prior art described above.
It is another object of the present invention to provide a high frequency signal source using a digitally compensated, frequency accurate, free running dielectric resonator oscillator circuit that overcomes the disadvantages of the prior art.
The present invention provides a novel and unobvious signal source using a digitally compensated, frequency accurate, free running dielectric resonator oscillator that resolves the shortcomings of traditional free running dielectric resonator oscillators. The circuit of the present invention provides a wide frequency tuning range, requires no manual tuning, and eliminates frequency drift over temperature and aging. The present invention can be used for generating a low cost, stable, and high frequency local oscillator source for terrestrial and satellite communications.
The high frequency signal source of the present invention generates a high frequency local oscillator signal using a free running dielectric resonator oscillator, which is compensated by a Phase Locked Loop (PLL), Voltage Controlled Oscillator (VCO), thus, eliminating the requirement for manual tuning of the dielectric resonator oscillator. The circuit of the present invention automatically compensates for dielectric resonator oscillator frequency drift over temperature and aging, and achieves a wide frequency tuning range without compromising phase noise performance. It eliminates manufacturing complexity, tuning and the costs associated with a voltage controlled dielectric resonant oscillator currently used by many skilled in the art. It also self-compensates for frequency drift caused by mechanical or printed circuit board (PCB) variation over temperature.
In accordance with the present invention, the high frequency signal source includes a dielectric resonator oscillator having an output signal. A mixer receives the output signal from the dielectric resonator oscillator. A phase locked loop circuit has a voltage controlled oscillator with a predetermined tuning range operatively connected to the mixer such that the mixer receives an output frequency from a voltage controlled oscillator and sums the output frequencies for creating a summed output frequency. A portion of the summed output frequency is fed as a coupled signal into the phase locked loop circuit that is phase locked to a reference signal, wherein a higher output frequency accuracy with low phase noise is achieved without manual tuning and a portion of the tuning range of the voltage controlled oscillator compensates for any dielectric resonator oscillator initial frequency error and drift over temperature and aging.
In yet another aspect of the present invention, a crystal reference oscillator is operatively connected to the phase locked loop circuit and provides a stable reference signal thereto. A filter is operatively connected to the mixer and filters the summed output frequency and eliminates unused side bands. The filter includes a high side filter that filters the upper side band of the summed output frequency and provides a final output frequency. A low side filter is operatively connected to the phase locked loop circuit and filters the lower side band of the summed output frequency and provides a coupled signal to the phase locked loop circuit.
An amplifier is operatively connected to the mixer and amplifies the summed output frequency. A divider circuit can be positioned within the phase locked loop circuit and divide the coupled signal by a factor xe2x80x9cNxe2x80x9d, respectively. A phase locked loop chip can have registers that are programmed for dividing the coupled signal and reference signal by a divide ratio xe2x80x9cMxe2x80x9d and xe2x80x9cNxe2x80x9d. A microcontroller is connected to the phase locked loop chip and can program the divide ratio between xe2x80x9cMxe2x80x9d and xe2x80x9cNxe2x80x9d.